In FM broadcasting, left and right stereo base band signals are low-pass filtered and combined to produce a composite stereo signal. The circuit that combines the left and right component signals and produces the composite stereo signal is called an exciter.
Once generated, the composite signal is used to drive an FM modulator which modulates a carrier wave in accordance with the composite signal. The modulated carrier wave is then broadcast using an FM antenna.
To be broadcast from an antenna, the modulated carrier wave must be an analog signal. For this reason, conventional systems have generated the composite stereo signal using analog equipment. However, there are a number of difficulties that arise in generating the composite stereo signal in the analog format. For example, low-pass filtering and sub-carrier stereo modulation are very complicated for an analog system. Mechanical filters may be used, but are large and bulky. Additionally, analog filters introduce phase distortions and group delay distortions into the resulting signal. These distortions are very difficult to correct.
The alternative is to generate the stereo composite signal in the digital format and then, eventually, convert the signal to an analog signal for broadcasting. With recent advances in the quality of digital signal processing hardware, including high speed, high precision A/D and D/A converters, an FM exciter using digital signal processing has a far superior performance than the counterpart analog system and costs much less.
FIG. 1 shows a typical digital signal processing system for a digital FM exciter. In FIG. 1, the left channel 101 provides a left analog audio signal which becomes the left component of the composite stereo signal. Similarly, the right channel 102 provides a right analog audio signal which becomes the right component of the composite stereo signal.
The left and right analog signals are respectively processed by anti-aliasing filters 104 and 105. After filtering, the left and right signals are respectively converted from analog into digital signals by A/D converters 107 and 108. The converted digital signals are provided to a digital signal processor (DSP) 109.
Generally speaking, the DSP 109 combines the left and right signals into a composite digital signal. More specifically, the DSP 109 performs band limiting filtering, pre-emphasizing, left and right channel mixing, sub-carrier generation, sub-carrier modulation and Sin(x)/x compensation for the D/A converter. Additionally, the DSP 109 provides soft level limiting (soft clipping), loudness signal monitoring for analog and digital automatic gain control, and spectrum analysis for optimized system control and operation.
The composite digital signal output by the DSP 109 is then converted to an analog signal by D/A converter 111 and filtered through low pass filter 150. The result is a composite analog base-band stereo signal 151 which may be used to modulate a carrier wave which is then broadcast by an FM antenna.
The drawbacks of this system result from the fact that the D/A converter 111 and the external analog FM modulator (not shown) must be of the highest quality, and therefore are very expensive. The high quality processing achieved by the front end A/D converters 107 and 108 and the DSP 109 will be lost if the D/A converter 111 and analog FM modulator (not shown) cannot match the performance of the DSP 109.
Accordingly, there is a need in the art for a system that digitally generates a high quality analog stereo signal without making excessive demands on the D/A converter and analog FM modulator which must receive and prepare the stereo signal for broadcasting.